Field of the Invention
The invention relates to a high temperature fuel cell, a high temperature fuel cell stack and a method for producing a high temperature fuel cell.
It is known that, during the electrolysis of water, the water molecules are decomposed by electrical current into hydrogen and oxygen. In fuel cells, that process takes place in reverse. During the electrochemical combination of hydrogen and oxygen into water, electrical current is produced, with high efficiency and without the emission of pollutants and carbon monoxide, when pure hydrogen is used as a combustible gas. Even with technical combustible gases, for example natural gas, and with air instead of pure oxygen, fuel cells produce considerably less pollutants and less CO.sub.2 per energy unit because of their high efficiency, than other forms of energy production which operate with fossil energy sources. The technical implementation of the principle of the fuel cell has given rise to a wide variety of solutions, namely with different electrolytes and with operating temperatures of between 80.degree. C. and 1000.degree. C.
In Solid Oxide Fuel Cells (SOFCs), natural gas is used as the primary energy source. The very compact structure permits a power density of 1 MW/m.sup.3. Operating temperatures of more than 90.degree. C. are found.
In a high temperature fuel cell stack being formed of high temperature solid electrolyte fuel cells, and also being abbreviated as merely "stack" in the specialist literature, a contact layer, a solid electrolyte electrode element, a further contact layer, a further interconnecting conducting plate, etc. are disposed in that order on one another and below an upper interconnecting conducting plate which covers the high temperature fuel cell stack. The electrolyte/electrode element in that case includes two electrodes and a solid electrolyte disposed between the two electrodes. The interconnecting conducting plates within the high temperature fuel cell stack are constructed therein as bipolar plates. In contrast to an interconnecting conducting plate disposed on the edge of the high temperature fuel cell stack, they are provided on both sides with channels for supplying the solid electrolyte electrode element with an operating medium.
In that case, a solid electrolyte electrode element lying between two neighbouring interconnecting conducting plates, inclusive of the contact layer bearing directly on both sides of the solid electrolyte electrode element and the sides of each of the two interconnecting conducting plates bearing on the contact layer, together form a high temperature fuel cell.
That and other types of fuel cells are, for example, disclosed by the "Fuel Cell Handbook" by A. J. Appelby and F. R. Foulkes, 1989, pages 440 to 454.
Experience has shown that an essential problem in the operation of a high temperature fuel cell is achieving longterm-stable electrical contact between the metallic interconnecting conducting plate and the electrodes of the fuel cell. To that end, use is made of so-called contact layers. To date, ceramic powders of the perovskite system (La, Sr) (Co, Mn)O.sub.3 have for that purpose been applied to the metallic interconnecting conducting plate in the form of a paste using a screen printing technique or in the form of a spraying suspension using wet powder spraying. Investigations of that material system have shown that, when Sr-doped material is used, a poorly conductive SrCrO.sub.4 layer is formed at the interface between the contact layer and the interconnecting conducting plate. Since the interconnecting conducting plate must be used as a current take-off, that poorly conductive contact layer is unacceptable.
A further problem, besides that of electrical conductivity, is the thermodynamic stability at the operating temperature of the high temperature fuel cell stack. Operating temperatures of more than 900.degree. C. place great demands in terms of thermodynamic stability. There is a further need to avoid undesired phase formation which arises due to the chemical reaction of elements that are present in the contact layer and not in the interconnecting conducting plate or the electrode.
An additional problem is the evaporation of chromium from the chromium-containing interconnecting conducting plate, which leads to degradation of the fuel cell.